Site Offline: Language + Usability patch

Fixes:
-Change offline to off-line. [Ref: offline and online]. This is probably a little contentious :)
-Site Off-line form now notifies the admin that authorized users can log in to the site using ?q=user during off-line mode. This is not mentioned anywhere else..
-Added a status message during off-line mode that is displayed on every page. Without this, logged in Administrators (besides the person who set the permission) will be completely unaware that the site is in offline mode.

The status message can probably be implemented in a neater fashion - ideas welcome :)

Thanks
-K